Metallized polymer sheeting is now commonly employed as a substitute for decorative chrome parts, especially in the automotive industry. Typically, such metallized polymer sheeting includes a layer of metal disposed between two polymer sheets.
There are several problems, however, with many types of known metallized polymer sheets. For example, laminates typically include an electrically continuous metal layer sandwiched between two polymer sheets. Such materials are often subject to delamination consequent to poor binding between the metal layer and the polymer layers on either side. Further, corrosion of the metal layer, which is usually aluminum can spread between the polymer layers, thereby causing significant diminution in appearance.
One attempt to reduce the likelihood of delamination and loss of appearance resulting from corrosion of the metal layer has been to form a discontinuous metal layer on a polymer basecoat, such as a resinous urethane. A monomer top-coat, such as a solvent-based aliphatic urethane, is then deposited on the discontinuous metal layer, and subsequently polymerized to encapsulate metal islands of the discontinuous metal layer and to bond them to the polymer basecoat.
However, formations of metal islands on various types of polymers can be difficult. Also, bonding of a urethane top layer during polymerization to a polyurethane basecoat can be poor. One attempt to improve adhesion has been to etch the basecoat and discontinuous metal layer with a sodium hydroxide solution to remove residual metal between islands of the discontinuous metal layer. A limitation to this method is that etching can result in the formation of blackened areas in the metal layer, thereby detracting from the appearance of the resulting laminated part.
There are several other problems that can be associated with polymerizing a top layer in situ to form metallized polymeric sheeting. For example, polyurethanes, in particular, generally are not sufficiently hydrophobic to prevent weathering over extended periods of time and are easily attacked by sodium hydroxide and acids, such as nitric acid. Thicker layers of polyurethane top-coat are difficult to form because in situ polymerization can cause the resulting composite to appear irregular. In addition, evaporation of a solvent component during polymerization of urethanes can cause xe2x80x9cpoppingxe2x80x9d or bubbles to form, also diminishing the appearance of the finished product. Further, methods which employ deposition of a basecoat, such as a resinous urethane basecoat, require that the basecoat be applied to a substrate, from which the resulting metallized composite generally cannot be removed. Therefore, the utility of this method for forming various products, having different applications, is limited.
Therefore, a need exists for a metallized composite and a method for forming such a metallized composite that overcomes or minimizes the above-referenced problems.
The present invention is directed to a metallized sheeting, such as a formable metallized plastic sheet, and a composite. The invention is also directed to a method for forming the sheeting and composite.
In one embodiment, the invention is a formable metallized plastic sheet which, upon molding, does not cause degradation of reflectivity of the metal sheet.
In another embodiment, metallized composite includes a first thermoplastic layer and a discontinuous layer on the first layer. The discontinuous layer is formed of discrete islands of metal in an adhesive. A second thermoplastic layer is disposed over the discontinuous layer, whereby the discontinuous layer is between said first and second thermoplastic layers.
In still another embodiment, the metallized sheeting includes a continuous thermoplastic sheet and at least one discontinuous layer of metal within said thermoplastic sheet.
The method includes depositing a metal on a first thermoplastic layer to form a discontinuous layer of the metal. A second thermoplastic layer is laminated onto the discontinuous layer.
The present invention has several advantages. For example, neither thermoplastic layer of the composite is polymerized in situ. Rather, the thermoplastic layers are laminated together to sandwich the discontinuous layer of metal islands in an adhesive bedding. Consequently, a wider variety of polymers can be employed to form the composite, thereby enabling greater opportunity for improving specific qualities of the composite and for tailoring construction of the composite for specific uses. For example, the choice of polymerized web materials can be selected for improved formation of discrete metal islands, such as by combining a particular metal with a polymer web that minimizes residual metal between metal islands. Alternatively, a polymer web can be selected that is preferably suitable for specific methods of metal deposition. By minimizing the amount of metal that remains between metal islands of the discontinuous layer, the need for etching can be significantly reduced or eliminated.
Further, because a top polymeric layer is not formed in situ, greater thicknesses can be employed without diminishing the appearance of the finished product, thereby improving resistance to environmental use conditions, such as weathering. In some instances, a plasma of unsaturated monomers, such as acrylates or methacrylates, may need to be polymerized on indium in vacuo; in such instances, the top layer would be added in another operation. Also, evaporation of solvents during polymerization is eliminated, thereby preventing xe2x80x9cpoppingxe2x80x9d and other potential processing problems. Moreover, a wider variety of methods of forming the composite can be employed, such as by depositing metal islands first on a thermoplastic drum surface, and subsequently transferring the metal islands to a first continuous thermoplastic web. A second thermoplastic web can then be applied over the discontinuous layer to form the composite. In other embodiments, the first and second thermoplastic webs can be bonded to each other by melting, use of an adhesive, or by compression. All of these processing options provide potential sources for reducing the cost of production and increasing overall product quality and productivity.
Different polymers can be employed for the two thermoplastic sheets, thereby further broadening the utility of the composites of the invention. In addition, neither the first nor the second thermoplastic web is bound to a substrate. Consequently, composites of the invention can be made to be flexible. Specific applications of flexible reflectors or mirrors can include adjustable rear-view mirrors for use in automobiles and as substitutes for conventional chrome-plated metal parts. In another embodiment, the composites can be molded after formation without degradation of the reflectivity of the discontinuous metal layer. Molding, such as embossing, for example, can provide an inexpensive means for incorporating a logo into flexible patches, such as can be applied to apparel, footwear, etc., that have the appearance of being perfectly reflective.
Other uses include in-mold decoration, blow molding and thermoforming. In-mold decoration, for example, includes injection molding a thermoplastic behind the sheet of composite to enable formation complex plastic art, such as parts having a reflective, mirror-like surface. The injection molding resin should be compatible with the first layer (the layer that will contact the molten injection molding resin). Preferably, the composition of the injection molding resin and the composition of the first layer of the composite will be the same; for example, injection molding a thermoplastic polyolefin (TPO) onto a composite which has as its first layer (facing the polymer melt) a TPO. Alternatively, the first thermoplastic layer and the injection molding resin should be compatible in the melt stage. An example of such a combination is a thermoplastic sheet of polycarbonate and an injection molding resin of polycarbonate-ABS blend.
Blow molding is similar to injection molding except that the molding resin is melted, extruded through a die and then blown with air or gas pressure against the walls of a mold cavity. In this case, a sheet of composite would be inserted into the mold and then the resin would be injected behind it.
In a thermoforming operation, the sheet is heated to soften it and then pushed into a cavity of a particular shape by a hot die surface. Vacuum forming is a similar process that also incorporates a vacuum to draw the softened sheet into the mold cavity as die pressure is applied to the opposite face.